When having two sorts on the same table with no temporary in-between
(e.g. first for DISTINCT, then for ORDER BY), they would both try to
store their buffers in table->sort.

Fix so that SortingIterator owns the buffers, so that the two sorts
have separate results; it doesn't really belong in TABLE anyway.

Change-Id: Ide5bfccc81e13dc657ef6c5f3ab36c3b2f5009c1